The invention relates to television signal transmitting equipment and more particularly to a system for correcting non-linearities by intermediate frequency pre-modulation.
In equipment for transmitting or re-transmitting television signals by common channel (in which the image and sound components are amplified simultaneously in the same power amplifier), at the present time provision is made for considerably reducing the amplitude non linearities of the signal amplified by the amplifier by pre-correcting the signal by means of a non-linearity generator providing amplitude pre-modulation of the high frequency signal to be transmitted, before amplification. During amplification, the nonlinearities generated by the amplifier are compensated by the non linearities generated by the non-linearity generator. With this correcting device, very good results are obtained, the intermodulation being improved by 12 to 14 decibels. This same device also provides a reduction by 12% of the cross-modulation, which is the transfer of the amplitude modulation of the image component to the sound carrier. But, the intermodulation and cross-modulation minima are not generally obtained under the best conditions. It is then not possible from this correcting device alone to minimize simultaneously the intermodulation and cross-modulation.
Moreover, the non-linearity of the common power amplifier further produces other defects of the different carriers which cannot be corrected by this device. This is the case for example for amplitude distortions of the type such as compression of the sync pulses on the image carrier ; it is also the case for differential gain variations at the frequency of the chrominance sub-carrier. Finally, there also exist other uncorrected defects of the incidential phase type, that is to say phase modulation of the image carrier as a function of its modulation level, i.e. of the level of the videofrequency signal.
As already mentioned, these defects have in common the fact that they occur at given frequencies in the composite signal spectrum and are a function of the videofrequency signal.
Furthermore, the common power amplifier introduces phase non linearities because of the non linearity of its phase-frequency characteristic. In television equipment, provision is also made for correcting these non linearities by using differential phase corrector devices, placed in the intermediate frequency stages, which are normally constructed so as not to introduce amplitude distortions. Patent application No. 7 35448 describes such a differential phase corrector device formed from correction cells which allow a variable phase shift to be introduced, about poles at frequencies selected by the corrector device, without affecting the amplitude of the signal. With such a device, phase distortions may for example be corrected which are introduced on to the chrominance carrier by the luminance signal.